1. Field of the Invention
This invention relates broadly to the field of optoelectronics devices, and, more particularly to optoelectronic circuits that convert optical signals into electrical signals and/or perform high speed signal sampling operations.
2. State of the Art
Optical networks provide the advantages of increased speed and transmission capacity for carrying voice and data. In optical networks, optical signals (e.g., light waves) are used to carry the information over the network. This information is provided by a source typically in electrical form and converted into an optical signal for transmission over the network.
In order to carry voice, the sound waves of the spoken voice are typically converted into an analog electrical signal which is converted into a digital electrical signal consisting of bits of information, wherein each bit is either a logic level ‘1’ (the amplitude of the digital electrical signal is high/ON) or logic level ‘0’ (the amplitude of the digital electrical signal is low/OFF). Data is typically stored in digital form as bits of information, and thus analog-to-digital conversion is not necessary.
Once the digital electrical signal has been obtained, it is converted to a digital optical signal by an electrical-to-optical converter, which modulates a laser light source in response to the digital electrical signal. The digital optical signal consists of bits of information, wherein each bit is either a logic level ‘1’ (the light intensity level of the digital optical signal is high/ON) or a logic level ‘0’ (the light intensity level of the digital optical signal is low/OFF). The digital optical signal is then transmitted over a medium (such as a light guide optical fiber).
An optical-to-electrical converter receives the digital optical signal produced by the laser source and transmitted over the medium, and generates a digital electrical signal corresponding to the digital optical signal. The digital electrical signal may be converted back to a digital optical signal for transmission over the optical network (such as the case where the optical-to-electrical converter is part of switch/router that operates in the electric domain on digital electrical signals). Alternatively, the digital electrical signal may be converted to an analog electrical signal (such as the case for voice applications where the analog electrical signal is converted back into sound waves that can be interpreted by the person receiving the phone call). Moreover, the digital electrical signal may be transformed for communication over a data communication link (such as the case for data applications where the digital electrical signal is transformed (e.g. packetized) for communication over a data communication link, such as a Gigabit Ethernet link).
FIG. 1A is a prior art functional block diagram illustrating a typical optical-to-electrical converter including a photodetector 110 (which may be one or more avalanche photodiodes or one or more PIN photodiodes) that converts the light level of the received digital optical signal to a signal current. The photodetector 110 delivers the extracted current to a transimpedance amplifier (TIA) 111, which first converts the current to a voltage. This single-ended voltage is amplified by the TIA and typically converted to a differential signal. A post amplifier 112 is provided, which in most cases is configured as a limiting amplifier that delivers a certain output-voltage swing whose maximum is independent of the input signal strength. A data recovery circuit 113 performs amplitude-level analysis on the signal output by post amplifier 112 to recover the serial digital data signal (in electrical form) from the received optical signal. Demultiplexing circuit 114 performs a serial-to-parallel conversion on the serial digital data stream generated by the data recovery circuit 113 to generate a multi-bit digital signal (electrical) representing a sequence of bits in the received digital optical signal.
The mechanism of FIG. 1A that converts the digital optical signal to a digital electrical signal (the photodetector 110, TIA 111, post amplifier 112 and data recovery circuit 113) is costly to design and manufacture because of the complex nature of the TIA 111, post amplifier 112 and data recovery circuit 113, and because of difficulties in integrating one or more of these components with the photodetector 110.
Thus, there is a great need in the art for an optoelectronic circuit that converts a digital optical signal to a digital electrical signal in a manner that has lower cost and improved ease of integration, and that is suitable for high speed applications.
The limitations of FIG. 1A are also present in parallel optical data links that have been developed to provide for increased aggregate data rates. As shown in prior art FIG. 1B, a parallel optical data link consists of a transmit module 120 coupled to a receive module 122 with a multi-fiber connector 124. The transmit module typically employs an array 126 of vertical-cavity-surface-emitting lasers (VCSELs) and a multi-channel laser driver integrated circuit 128 for driving the array of lasers to produced a plurality of synchronous optical bit streams that are transmitted over the multi-fiber connector 124. The receive module 122 includes a photodetector array 130 (typically realized with P-I-N diodes) that receives the synchronous optical bit streams and cooperate with an integrated circuit 132 that provides a corresponding array of low noise transimpedance amplifiers, limiting amplifiers, and data recovery circuits to produce a plurality of electrical bit streams corresponding thereto. The plurality of electrical bit streams are provided to one or more integrated circuits 134 that map parallel bits encoded in the plurality of electrical bit streams into a predetermined data format (such as a SONET frame). Although such a parallel optical data link provides cost savings based upon array-integration of electronic and optoelectronic components in both the transmit module and the receive module, it suffers from the same limitations of the approach of FIG. 1A. The complex nature of the TIA, post amplifier and data recovery circuit in the receive module 122 leads to increased design costs and manufacture costs of the receive module 122, and also leads to difficulties in integrating one or more of these components with the photodetector array as part of the receive module 122.
Thus, there is a great need in the art for an optoelectronic circuit that converts a plurality of synchronous optical bit streams to electrical bit streams in a manner that has lower cost and improved ease of integration, and that is suitable for use in high speed applications (such as the receive module of a parallel optical data link).
For high frequency applications, optoelectronic integrated circuits that convert an optical signal to an electric signal have been reported. For example, Dutta et al., “10 GHz bandwidth monolithic p-i-n modulation-doped field effect transistor photoreceiver,” Appl. Phys. Lett., Vol. 63, No. 15, October 1993, pp. 2115–2116, describes the use an InGaAs PIN photodiode for the conversion of incident photons to electrons followed by an amplifier circuit based on a modulation-doped field effect transistor. And Akahori et al., “10-GB/s High-Speed Monolithically Integrated Photoreceiver Using InGaAs p-i-n PD and Planar Doped InAlAs/InGaAs HEMT's,” IEEE Photonics Technology Letters, Vol. 4, No. 7, July 1992, pp. 754–756 describes the use of an InGaAs PIN photodiode for the conversion of incident photons to electrons followed by an amplifier circuit based on planar doped InAlAs/InGaAs HEMT devices. And Hurm. et. al., “20 Gbit/s long wavelength monolithic integrated photoreceiver grown on GaAs,” Electronics Letters, Vol. 33, No. 7, 1997, pp. 624–626, describes the use an MSM photodiode for the conversion of incident photons to electrons followed by an amplifier circuit based on an AlGaAs/GaAs HEMT transistor. However, these prior art mechanisms require substantially different epitaxial growth structures to realize the components of the optoelectronic integrated circuit, and thus are costly to design and manufacture.
As described above, digital optical signals may be used to carry analog information (such as voice). In such applications, it is necessary that the optical bits encoded in the digital optical signal be converted into an analog electrical signal for subsequent processing. Prior art FIG. 1C is a functional block diagram illustrating a typical mechanism for performing such conversion operations. Similar to FIG. 1A, a photodetector 110 converts the light level of the received digital optical signal to a signal current. The photodetector 110 delivers the extracted current to a transimpedance amplifier (TIA) 111, which first converts the current to a voltage. This single-ended voltage is amplified by the TIA and typically converted to a differential signal. A post amplifier 112 is provided, which in most cases is configured as a limiting amplifier that delivers a certain output-voltage swing whose maximum is independent of the input signal strength. A data recovery circuit 113 performs amplitude-level analysis on the signal output by post amplifier 112 to recover the serial digital data signal (in electrical form) from the received optical signal. Demultiplexing circuit 114 performs a serial-to-parallel conversion on the serial digital data stream generated by the data recovery circuit 113 to generate a multi-bit digital signal (electrical) representing a sequence of bits in the received digital optical signal. The multi-bit digital signal produced by the demultiplexing circuit 114 is provided to a digital-to-analog converter 115 that converts the multi-bit digital signal to a corresponding analog electrical signal. This approach suffers from the same limitations of the approach of FIG. 1A, wherein the complex nature of the TIA, post amplifier and data recovery circuit leads to increased design costs and manufacture costs. In addition, the large number of complex components that make up the signal processing chain (from photodetector 110 to the digital-to-analog converter 115) are costly to design and manufacture.
Thus, there is a great need in the art for an optoelectronic circuit that converts a digital optical signal to an analog electrical signal in a manner that has lower cost and improved ease of integration, and that is suitable for high speed applications.
Similarly, a parallel optical data link may be used to carry analog information (such as voice). In such applications, it is necessary that the optical bits encoded in the digital optical signals be converted into an analog electrical signal for subsequent processing. This approach suffers from the same limitations of the approach of FIGS. 1A and 1B, wherein the complex nature of the TIA, post amplifier and data recovery circuit leads to increased design costs and manufacture costs. In addition, the large number of complex components that make up the signal processing chain (from photodetector to the digital-to-analog converter) are costly to design and manufacture.
Thus, there is a great need in the art for an optoelectronic circuit that converts parallel optical bit streams to an analog electrical signal in a manner that has lower cost and improved ease of integration and that is suitable for high speed applications.
In addition, digital-to-analog converters (and other signal processing circuitry such as analog-to-digital converters, switched-capacitance filters/amplifiers, and switched-capacitance waveform generators) typically employ electrically-controlled transistors as on-off switches to perform signal sampling operations. Due to parasitic capacitance and intrinsic capacitances between the input and output nodes of the sampling transistor, feedthrough charge that collects on the sampled signal increases to an intolerable level at high frequencies. Therefore, the electronic sampling technique becomes limited in sensitivity at high sampling rates.
Thus, there is a great need in the art for improved signal sampling mechanisms that are suitable for high sampling rates and avoid the limitations (including feedthrough charge) of the prior art transistor-based sampling mechanisms.